1. Field of the Invention
The present invention relates generally to a sensor for detecting the presence of an object and, more particularly, to a sensor which comprises an amplifier and a means for dynamically changing the gain of the amplifier in response to changing conditions.
2. Description of the Prior Art
Many types of sensors are known to those skilled in the art. One particular type of sensor is a photoelectric sensor that comprises both a light source and a light sensitive component although other types of radiation can also be used. The light source can be received by the light sensitive component according to any one of a number of procedures that are well known to those skilled in the art. The light beam that is transmitted from the light source to the light sensitive component is disposed through a region where objects are expected to move. When an object breaks or affects the light beam passing from the light source to the light sensitive component, the presence of the object is detected. When solid objects are being detected by a photoelectric sensor, a fixed threshold magnitude can be used to compare with the intensity of light received by the light sensitive component and thereby detects the attenuation of the light beam as a result of the presence of an object between the light source and light sensitive component. When transparent or nearly transparent objects are to be detected, the use of a fixed threshold magnitude is often unsatisfactory because of the very slight change in the intensity of the light beam that occurs when it passes through the transparent or nearly transparent object. In certain applications where transparent objects are to be detected, such as in the detection of glass or plastic bottles, the decrease in the intensity of the light beam is so small that the effectiveness of the photoelectric sensor can be seriously diminished by changing conditions such as temperature, age or mist in the region of the light beam. These changing conditions can be caused by a change in the ambient conditions or a degradation in the operation of the light source. If, for example, an LED is used as the light source, aging or changes in temperature can seriously affect the intensity of light transmitted by the light emitting diode. This decrease in intensity of the light beam emitted by the light emitting diode is difficult to distinguish from the decrease in light intensity received by the light sensitive component when a transparent or nearly transparent object is attenuating the beam.
In view of these problems that can occur in certain applications of photoelectric sensors, it would therefore be significantly beneficial if an efficient means were provided to adjust the sensitivity of the detection system in response to changes in environmental conditions or the intensity of light emitted by the light source.